UC Davis

Next generation backside imager photodiodes are presently being developed for use in hard x-ray detection and imaging at the National Ignition Facility (NIF) at Lawrence Livermore National Lab (LLNL). NIFs mission is to establish nuclear fusion capabilities, and accurate high-speed diagnostics play a critical role in advancing research. Replacing current silicon-based photodiodes with germanium-based technology allows for utilization of germanium’s improved intrinsic semiconductor properties to create diodes that are thinner and faster. Creating a photodiode with optimized speed and detection limits requires the backside imager geometry in which the substrate is removed, and the bottom of the wafer is used for photon collection. Established silicon etching techniques cannot be applied to germanium due to different fundamental chemical properties thus the selective removal of a germanium substrate is a yet unsolved processing problem.This research project has investigated the use of intentional doping of germanium layers to control and effectively accelerate or arrest electrochemical etching. Research activities were separated into three sections: development of a system and methodology for investigating the electrolytic etch process for germanium, identification of a suitable etchant system, and demonstration of etch selectivity within the determined system. The major result of this PhD project is that under anodic etching conditions with an applied bias of -2V, high-purity intrinsic germanium etches marginally whereas etching of highly boron-doped germanium is accelerated by at least 3 orders of magnitude. A potential processing methodology is proposed that will enable manufacturing of germanium-based photodiodes for future backside imaging devices.